The present invention relates to the technical field of telescopic booms or jibs for cranes, whether installed on fixed installations or carried by mobile structures, whether self-propelled or not.
In the above technical field, telescopic jibs are generally mounted on a turntable that is steerable in azimuth relative to a carrier structure, and the turntable carries a jib that is tiltable in elevation, the jib being made up of at least three distinct telescopic elements.
The first element is referred to as a xe2x80x9cbasexe2x80x9d element and is generally connected to the elevation tilt axis, the second element is referred to as an xe2x80x9cintermediatexe2x80x9d element, and the third element is referred to as an xe2x80x9cendxe2x80x9d element and usually carries at least one jib head sheave. These three elements are intended to enable the length of the jib to be adjusted, so as to vary the distance or length between the elevation adjustment axis and the jib head sheave over which a cable passes and is guided, which cable is wound in or out from a base winch in order to vary the height of a load suspended from the portion of the cable that extends beyond the jib head sheave.
Initially, implementing a jib of the above type required the existence of technical means suitable for making it possible to adjust telescoping while under load so as to make it possible to adjust jib length relative to the vertical through a determined point relative to which a load was to be lifted, lowered, put into position, or removed.
To be able to perform such a function, it was necessary to provide relative sliding means between the telescopic elements capable of accommodating the stresses that arise while telescoping under load, and it was necessary to implement telescoping means that were particularly powerful in order to overcome the friction induced by sliding under load.
Such means are particularly expensive, they must be designed to be very robust, and they need to be subjected to very thorough maintenance so that the essential or prime function of the crane can be carried out in complete safety. Furthermore, such means represent considerable weight to the detriment of the functional capacity of the jib.
For some time, attempts have been made to emphasize prior telescoping while unloaded, while nevertheless still having the possibility of enabling the stroke of one element to be telescoped under load. Such a method makes it possible to install telescoping power that enables the unloaded jib to be set to length while nevertheless retaining the possibility of modifying said length over the stroke of a single element at reduced load. That method has the advantage of not straining the locking and unlocking members.
That technique consists in determining a priori the length that needs to be conferred to the jib so that from the site on which it stands it can reach a position vertically above the point where a load is to be picked up or put down, or indeed a locus which this point is capable of following, e.g. a portion of a circle, and then in adjusting the length of the jib accordingly.
To enable those techniques to be implemented, an actuator is placed inside a telescopic jib, which actuator is suitable for performing rectilinear displacement in both directions, and is connected, for example, to the base element and movable relative to the end element.
In general, such an actuator is constituted by a double-acting hydraulic actuator of single predetermined stroke which corresponds generally to the longest retraction or extension stroke that can be performed by any of the moving elements, naturally excluding the base element which is considered as being fixed.
The above technical means need to satisfy two basic requirements.
The first is to be able to hold the telescopic elements in a mutually locked configuration both in the retracted position and in the extended position.
The second is the need to be able to engage selectively by any one of the telescopic elements to release it from the position it is occupying and then drive it to a desired new position in which it must again be locked.
To make that possible, the prior art has various proposals.
Mention can be made of application EP 0 476 225 which mentions implementing a telescoping head carried by the actuator and suitable for causing locking blocks to be retracted or extended relative to complementary housings presented by the internal ends of the telescopic elements which are also interconnected in the retracted position by other selectively controllable members which need to be actuated synchronously.
That document would not appear to specify how the elements are caused to be locked together in the extended position.
In any event, it should be observed that that technology uses a series of locking members and engagement members which are superposed and which need to be fed with energy in the form of a fluid under pressure, which energy must be delivered in accurately synchronized manner in order to obtain the desired result which, by way of example, when performing outward telescoping, necessarily requires the following to be performed in sequence: the element concerned is engaged; the element is unlocked; the element is caused to slide relative to the others into its new position; in which position it must again be locked.
Mention can also be made of German application DE 4 344 795 in which, between the actuator and each of the telescopic elements, there are provided engagement means of the same kind as in the above-described application, and locking means which are constituted by engaging pins that need to be controlled separately during retraction in order to release one element relative to another.
Although that technology appears to be technically simpler than the technology of the preceding application, it nevertheless requires two types of control member to be implemented that are capable selectively of connecting the actuator to any one of the elements to be displaced, and of releasing said element that is to be displaced relative to the next element.
Such a requirement means that specific control circuits need to be established and moving members need to be present that are specific to each of those functions, and as a result, overall the telescoping apparatus is cumbersome to install, to put into operation, and to maintain, and it is not possible for it to claim any genuine savings in weight or mass compared with the solution proposed in application EP 0 476 225.
The object of the invention is to propose a novel telescoping technique and novel means for implementing it so as to be able to perform the above functions in complete safety while using equipment of reduced weight. An object of the invention is to be able to transfer the weight saving obtained in this way to the benefit of jib construction, thereby enabling the effectiveness of the jibs to be reinforced without it being possible overall to consider that the total mass has changed to such an extent as to reduce functional capacity.
The invention proposes a novel technique and novel implementation means making it possible from a combination of two mutually co-operating technical means to perform both functions comprising firstly causing the actuator to operate in temporary manner to engage or to couple with one of the telescopic elements in order to apply thrust thereto, and secondly to release the telescopic element from the element which surrounds it so as to allow relative displacement between the elements, and conversely the opposite functions when it is appropriate to lock in the desired new position any one of the elements that have been moved so that it can then be decoupled from the actuator.
To achieve the above objectives, the method of the invention consists in:
implementing on each element other than the base element a locking block including a clamping member having a clamped rest position;
causing the clamping member of each block to co-operate with at least one fixed-position immobilization piece belonging to the element disposed concentrically outside the element carrying the block; and
causing the actuator to carry a member suitable for acting selectively on each of the locking blocks for the purposes of temporarily coupling or engaging them and for positively unclamping them;
by positive action firstly to connect said actuator to said block and secondly to neutralize the action of the clamping member on the immobilization piece; and
by retraction firstly to immobilize said block relative to the clamping piece and secondly to release the actuator relative to said block.
The invention also provides telescoping apparatus capable of implementing the above method, wherein:
the locking members are constituted by:
a locking block carried by each element other than the base element, and including a clamping member having a clamped rest position; and
at least one immobilization piece of fixed position belonging to the outer element disposed concentrically about the element carrying a block and designed to co-operate with the clamping member of said block;
and the driver means on the actuator comprise a single engagement member suitable:
for selectively engaging any one of the blocks to couple the actuator temporarily to the element carrying said block; and
for taking positive action to neutralize the clamping member of said block so as to release it relative to the immobilization piece.
The invention also provides a jib implementing the above means.